This 40 yard bullshit is a curiously American phenomenon. It's also a typical American tactic. If you can't compete at something invent your own version and declare yourself champion. What use would a 40 yard dash speed be to someone playing football or rugby? There is literally no sport in the world apart from rugby-for-cross-dressers in which being able to run quickly in a straight line for forty yards is any sort of advantage.

40 yard dash is just to measure the speed of the players compared to others in football, but it is also a good hint at how fast they are. Most fast football players had a track and field career in high school and college but decided against it because football or basketball is more lucrative. The track numbers are pretty easy to find Jackson for example ran the 100 meter in 10.39 in high school.